Plasma processing techniques are used to deposit films on the surface of substrates or to modify films on the substrates. These techniques are widely used, for example, in the semiconductor or flat panel industries to fabricate integrated circuits. During semiconductor processing, a substrate such as a silicon wafer is placed in a processing reactor and chemical vapor deposition (CVD) techniques may be used to deposit a layer of material on the surface of the wafer. A plasma environment may be utilized to provide plasma enhanced CVD (PECVD). Additionally, an etching process may be employed where a previously deposited film is modified in some desired manner, usually to form circuit components or interconnect lines.
During such processing the wafer is often supported in the reactor by a chuck. Various chuck designs are well known in the art. One type of chuck support often used with PECVD techniques is an electrostatic chuck. An electrostatic chuck generally includes at least one electrode and a thin dielectric coating across the surface of the electrode. The wafer is electrostatically clamped to the chuck by applying a dc voltage to the electrode. This causes a voltage to develop across the dielectric and image charges are induced on the wafer. The opposite charges attract the wafer to the chuck, clamping the entire wafer to the chuck surface with a force referred to as an "electrostatic force" or a "clamping force." This clamping force is proportional to the square of the voltage and inversely proportional to the separation between the electrodes of the chuck and the wafer. If the voltage or the nature of the dielectric varies across the chuck, the clamping force will vary as well. One example of an electrostatic chuck is fully described in co-pending U.S. application Ser. No. 08/500,480 (Attorney file no. A-62195/AJT/JEM) which is incorporated herein by reference.
During processing, temperature gradients develop across the wafer and may cause non-uniform processing, such as variations in the deposition rate which adversely affects the film quality, or may even damage the underlying integrated circuit. Consequently, it is desirable to control the temperature of the surface of the wafer. Temperature control may be accomplished by placing the backside (the surface where no processing occurs) of the wafer in thermal contact with a temperature control medium. Typically, a gas such as helium is used, and is introduced between the backside of the wafer and the surface of the chuck support. The gas conducts heat between the two surfaces and can be utilized to either heat or cool the wafer as process conditions warrant. Heat transfer improves with gas pressure.
The presence of the gas medium creates certain problems. The force generated by the pressure of the gas between the wafer and the chuck surface is generally greater than the weight of the wafer, and thus the gas tends to push the wafer off the chuck. Consequently, the electrostatic force securing the wafer must be great enough to overcome this opposite force.
During processing, it is often desired to sputter etch the surface of the wafer with ions. Sputter etching may take place during a deposition step to produce good gap fill in submicron dimension gaps, and to enhance the deposition of void-free, dense, conformal films. To sputter etch the surface of the wafer, an rf bias is applied to the wafer which causes the acceleration of ions from the plasma sheath present in the reactor toward the surface of the wafer.
Due to the nature of the plasma, any object which is not at ground and is placed in the plasma environment will acquire a net negative charge. This causes an "induced dc voltage" to be established on the surface of the wafer exposed to the plasma. The induced dc voltage is defined as the time average of the voltage on the wafer surface exposed to the plasma. Additionally, when rf power is applied across a capacitor (as in the case where rf bias is applied to the chuck and dielectric to induce sputter etching), the voltage of the surface of an object exposed to the plasma will become more negative. Consequently, with the application of rf bias, the induced dc voltage on the wafer is changed, and most importantly, the clamping force securing the wafer is also changed. This change in clamping force results in non-uniform processing of the wafer, temperature gradients across the wafer surface, and in the worse case, declamping of the wafer causing a catastrophic failure of the process and arcing within the reactor.